What would be the best way to add groves to this control knob?
One easy way to do it is to use the Simple Deform modifier / Bend mode:

Create your segment:



Array it (enable the Merge option of the Array modifier):



Give it the Simple Deform modifier / Bend mode:



You can continue to work on the segment as much as you want, when you're glad with the result, apply the Array and Simple Deform modifiers (keep the Subdivision Surface if possible), merge the vertices: